RFID interrogation systems have been proposed to detect the presence of an RFID tag associated with an article of interest. An example of such a system is disclosed in Patent Cooperation Treaty (PCT) Publication WO 99/05660, which is assigned to Checkpoint Systems Inc. That publication describes the use of an RFID inventory system in which the materials in a library can be checked in and out by detecting the RFID tags associated with those materials. The publication further describes periodically scanning the shelves of the library with an RFID scanner for updating the inventory status of the materials.